1. Field of the Invention
The present invention relates to a thermal printer.
2. Description of the Related Art
Thermal printers of various types are provided at present in which printing is performed by pressing a heated thermal head against a special recording sheet which undergoes a color change when heat is applied thereto. In particular, the thermal printer is preferably used in printing variety of labels, receipts, and tickets because it is possible to perform the printing of smooth letters and various graphics without using toners, inks, or the like.
In general, the thermal printer includes a thermal head having a plurality of heat-generating elements, a platen roller for sandwiching the recording sheet with the thermal head, a motor for feeding the recording sheet by rotating the platen roller through a gear, and biasing members (such as coil spring or leaf spring) for pressing the thermal head against the recording sheet and the platen roller side. Further, the recording sheet used for the thermal printer is normally used in a state of being wound so as to have a hollow into roll paper.
For performing printing with the thermal printer described above, the thermal head is pressed against the recording sheet while the platen roller is being rotated by the motor to feed the recording sheet. Then, the heat-generating elements of the thermal head are operated based on information to be printed, thereby printing characters or graphics according to the information on the recording sheet passing between the thermal head and the platen roller. As a result, various types of printing as described above can be performed.
Incidentally, in the case where the thermal printer described above is incorporated into a portable information terminal or the like, a structure, in which the thermal head is provided on the side of a case for accommodating the roll paper therein, whereas the platen roller is provided on the side of a cover (lid member) capable of opening and closing an opening portion of the case (a so-called platen-open type structure), is common to easily sandwich the recording sheet pulled out from the roll paper between the thermal head and the platen roller when the roll paper is placed. Further, as described in patent document JP 2003-200624 A, for example, there is known a structure, in which lock arms for axially supporting the platen roller in a rotatable and attachable/removable manner are provided on side wall portions of a frame.
Here, for locking the platen roller provided on the cover side to the frame side, both end portions of a shaft of the platen roller must be locked to the lock arms when the cover is closed. However, when only one end side or the another end side of the cover in a width direction is pushed down, specifically, the cover corresponding to only one axial side of the platen roller is pushed down for closing the cover, a push-down force acts only on one side of the platen roller to bring about a so-called one-side locked state where only one side of the platen roller is locked. If the thermal printer is operated in this state, the recording sheet cannot be fed by the platen roller because the platen roller and the thermal head are not in even contact with each other. As a result, print processing is implemented in a state where precise printing cannot be performed on the recording sheet, and hence print information disappears to prevent the printing on the recording sheet.